1. Field of the Invention
The present invention generally relates to improving the uniformity of signals generated by a light sensor over a scan of a target and, more particularly, to collecting different amounts of light reflected off a bar code symbol from different regions of the scan, and directing such different amounts to a light detector for conversion into electrical signals whose amplitudes are more uniform over the scan of the symbol.
2. Description of the Related Art
In the field of electro-optically reading bar code symbols, it is well known to sweep a light beam across a symbol for reflection therefrom, to scan the beam and/or a field of view of a light detector in a scan across the symbol, to detect light reflected off the symbol with the detector, to convert the detected light into an electrical signal, and to process the signal into data related to the symbol.
The amplitude of the electrical signal generated by the detector is a function of several factors. For example, as described in U.S. Pat. No. 4,409,470, the amplitude is a function of the distance at which the symbol is located relative to the sensor. A symbol that is located closer to the sensor reflects more light to the sensor and, hence, the electrical signal has a larger amplitude as compared to a symbol that is further away from the sensor. The amplitude variation between such close-in and far-out symbols could be electronically compensated, but, as described in said patent, could also be optically compensated, thereby minimizing the variation for symbols located anywhere in a depth of field of a bar code reader.
The amplitude of the electrical signal generated by the detector is also a function of scan angle or location along the scan. Thus, in the case where a laser beam is swept along a single scan line across a symbol located at a given distance relative to the detector, the amount of reflected light collected from the end regions of the scan line is lower than the amount of reflected light collected from an intermediate region of the scan line between the end regions. Thus, the electrical signal generated by the detector is lower in amplitude for light reflected from the end regions of the scan line, and is higher in amplitude for light reflected from the intermediate region.
Put another way, the detector has an optical axis, and the light reflected from the end regions travels along a direction that is inclined relative to the axis, and the light reflected from the intermediate region travels along a direction that is generally parallel to the axis. The light from the end regions, also called xe2x80x9coff-axisxe2x80x9d light, has a greater scan angle than the light from the intermediate region, also called xe2x80x9con-axisxe2x80x9d light. Thus, the amplitude of the electrical signal generated by the detector is also a function of scan angle and varies over the scan.
In order to achieve reliable performance of the bar code symbol reader, the variation of the electrical signal versus scan angle must be minimized. Without signal uniformity, complex and expensive electronic compensation circuitry is needed and, if not provided, will significantly limit reader performance.
Accordingly, it is a general object of this invention to improve the performance of a bar code symbol reader.
More particularly, it is an object of the present invention to minimize electrical signal amplitude variation as a function of scan angle.
Still another object of the present invention is to collect light in different amounts from a target over a scan and to distribute the collected light to a sensor in order to render the signal variation more uniform.
In keeping with the above objects and others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in an optical collector for collecting light reflected from, and scanned in a scan across, a target, for example, a bar code symbol. The collector is used with a light sensor, for example, a photodiode, having an optical axis and operative for sensing incident light and for converting the sensed light to an electrical signal indicative of the symbol.
The collector includes first and second collecting portions preferably integrated into a single component. The first portion collects relatively more of the light reflected along directions that are generally inclined relative to the axis, i.e., xe2x80x9coff-axisxe2x80x9d light traveling along a non-zero or relatively larger scan angle, and directs the off-axis light to the sensor. The second portion collects relatively less of the light reflected along directions that are generally parallel relative to the axis, i.e., xe2x80x9con-axisxe2x80x9d light traveling along a zero or relatively smaller scan angle, and directs the on-axis light to the sensor. The off-axis light originates from spaced-apart end regions of the scan. The on-axis light originates from an intermediate region of the scan between the end regions thereof.
The amplitude of the electrical signal generated by the sensor varies with the scan angle. Typically, the amplitude varies as the fourth power of the cosine of the scan angle. Hence, according to the prior art, the signal generated from off-axis light is weakest at the ends of the scan where the scan angle is greatest, and the signal generated from on-axis light is strongest at the middle of the scan where the scan angle is a minimum.
In accordance with this invention, by collecting more of the light from the ends of the scan, the amplitude of the signal from the ends of the scan is increased, and by collecting less of the light from the middle of the scan, the amplitude of the signal from the middle if the scan is decreased. The result is that the signal amplitude variation over the scan is minimized. Reader performance is enhanced and rendered more reliable without having to resort to complex electronic compensation circuitry.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.